The properties of doped thin films depend not only on the dopant concentration, but also on the position, bonding, and spatial distribution of the dopants, which control the fraction of active dopants (doping efficiency). Thin film growth techniques such as chemical vapor deposition, sputtering or evaporation lack the ability of actively controlling the local environment of the dopant.
Atomic layer deposition (ALD) is a thin film growth method using alternating, self limiting reactions between gaseous precursors and a solid surface to deposit materials monolayer-by-monolayer. Due to its self-limited layer-by-layer nature, ALD provides a partial degree of control of dopant distribution in the direction of growth. The amount of dopant per layer is controlled by the saturation coverage, which is a given value for every precursor/coreactant combination. This leads to strongly inhomogeneous doping profiles for low dopant concentrations. Previous reported strategies to allow more doping control include searching for precursors with lower growth rate per cycle, and operating below the saturation coverage at the expense of losing the homogeneity and conformality of ALD. Consequently, substantial problems currently prevail with use of ALD for growth of metal oxides, metal sulfides, and other materials with prescribed dopant profiles.